history of genetics mendelian genetics punnett …

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• H I S T O R Y O F G E N E T I C S • M E N D E L I A N G E N E T I C S • P U N N E T T S Q U A R E S

• P E D I G R E E S

Genetics A

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  Genetic Disorder Quiz   History of Genetics & Gregor Mendel   Homozygous/Heterozygous Activity

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Genetic Terms

  Genetics: the study of inheritance   Trait: characteristic that can be passed from parent

to child   Ex. Hair color, Eye color, etc.   Ex. Pea Plants: seed shape, height, flower color, etc.

History of Genetics

  In the past, people explained hereditary by saying that traits from parents were “blended.”   Blending Hypothesis

Mom’s Genes Dad’s Genes

Child’s Genes

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History of Genetics

  We now know that during cell division, chromosomes are distributed through gametes.

Gregor Mendel

  “Father of Genetics”   Austrian monk   Began work in 1860’s

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Mendel’s Experiments

  Used 20,000 pea plants to study how traits are passed from one generation to the next

  Found that his results DID NOT support the “blending hypothesis”

Purebred Hybrid   Organism with the same

genetic info from parents   In Pea Plants, this is called:

  Self-fertilization   True breeding   (Sperm and egg from

SAME plant)   Also called Homozygous

  “GG” or “gg”

  Organism with different forms of a genetic trait from each parent

  In Pea Plants, this is called:   Crossbreeding   (Sperm and egg from

DIFFERENT plants)   Also called Heterozygous

  “Gg”


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Purebred Hybrid


ONE Plant TWO Plants

Mendel’s Observations

  Started by crossing 2 different PUREBRED plants by cross pollination   He called the 2 purebred plants PARENTS, or the

P Generation

Parents -OR-

P Generation P P

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  1st Generation of offspring, which was produced by crossing the 2 purebred parents, is called the F1 Generation.   All hybrid offspring only had the trait of ONE of the

parents (NO BLENDING!)

P Generation

F1 Generation

P P

F1

*Hybrid Cross


  Mendel allowed the F1 plants to self-fertilize to produce the F2 generation

F1 F1

F2 F2 F2 F2

F1 Generation

F2 Generation

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Generation Crossed Pea Color Crossed

Offspring Produced (Results)

P x P Yellow x Green All Yellow (F1 Generation Produced)

F1 x F1 Yellow x Yellow 3 Yellow, 1 Green (F2 Generation Produced)

F2 x F2 Differs Differs

Group Discussion

  Why do you think the P Generation made ALL Yellow peas?

  Why do you think the F1 Generation made 3 Yellow peas and 1 Green pea?

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  Repeated experiments for 6 different traits:   Flower color (purple/white)   Flower position (axial/terminal)   Pea shape (round/wrinkled)   Pod color (green/yellow)   Pod shape (inflated/constricted)   Height (tall/short)

  Defined each form has dominant (75% of F2) or recessive (25% of F2)

Gene

  Gene: section of a chromosome that codes for a trait   Allele: different forms of a gene

  Usually two different forms   Represented by a letter

 Ex. B = brown eye allele, b = blue eye allele   Two alleles make up a gene

  Dominant   Recessive

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Dominant Allele Recessive Allele

  Form of allele that is expressed (shown) when different alleles are present

  Always represented by a CAPITAL letter

  Example:   T = tall plant   P = purple flowers

  NOT expressed when the dominant allele is present

  ONLY present when both are recessive

  Always represented by a lower case letter that is the SAME as the letter that represents the dominant allele

  Example:   t = short plant   p = white flowers

Gene

Genotype

  Genetic make-up of an organism   Includes both genes in a homologous pair of chromosomes   In the form of LETTERS which represent genes   Example:

  Bb   PP   pp

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Phenotype

  Physical appearance of a trait   In the form of WORDS that describe the trait   Example:

  Green eyes   Button nose   Yellow pea color

Genotype Phenotype

Homozygous Dominant TT Tall

(dominant)

Homozygous Recessive tt Short

(recessive)

Heterozygous Tt Tall (hybrid)

*Sometimes this could be a mix of two traits*

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For the fol lowing Quest ions:

Purple (P) = Dominant White (p) = Recessive

Whiteboard Activity

Question #1

What is the GENOTYPE for HOMOZYGOUS RECESSIVE?

Purple (P); White (p)

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Question #2

What is the GENOTYPE for HETEROZYGOUS?


Question #3

What is the PHENOTYPE for HOMOZYGOUS DOMINANT?


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Question #4

What is the GENOTYPE for HOMOZYGOUS DOMINANT?


Question #5

What is the PHENOTYPE for HOMOZYGOUS RECESSIVE?


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Question #6

What is the PHENOTYPE for HETEROZYGOUS?


1.   L A W O F S E G R E G A T I O N 2.   L A W O F I N D E P E N D E N T A S S O R T M E N T

3.   L A W O F D O M I N A N C E

Mendel’s Laws

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1. Law of Segregation

  When an individual produces gametes, the copies of a gene separate so that each gamete receives only one copy   pg. 265-266

  Only one characteristic can be found in a gamete   Example:

  Purple flower vs. White flower (can only have one trait)   Blue eyes vs. Brown eyes

2. Law of Independent Assortment

  Alleles of different genes assort independently of one another during gamete formation, Meiosis   pg. 270-271

  For two characteristics, the genes are inherited independently of one another

  Also called the “Inheritance Law”   Example:

  If you had the GENOTYPE AaBb you would make 4 kinds of gametes and they would contain the combination of either:  AB, Ab, aB, or ab

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3. Law of Dominance

  Some alleles are dominant and some are recessive   Pg. 264-265

  When an organism has two different alleles for a trait (a hybrid or heterozygous individual), the allele that is expressed is the dominant one

  Example:   Round vs. Wrinkled seed (Rr = Round is expressed,

therefore it is dominant)

G R I D U S E D F O R O R G A N I Z I N G G E N E T I C I N F O A N D M A K I N G P R E D I C T I O N S

Punnett Squares

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One Trait (Monohybrid)

  When studying the inheritance of only one trait, called a monohybrid cross.

  For the first examples, we will only be testing the complete dominance condition.   When one allele completely dominates over the other.

Rules for Monohybrid Crosses

1.  Determine parent genotypes (CAPITAL = DOMINANT; lower case = recessive)

2.  Segregate alleles and place alleles of each parent on top and side of four-squared grid

a)  Mom on left side; Dad on top of grid 3.  Combine parent alleles inside boxes

a)  Letters inside boxes show POSSIBLE genotypes of offspring – not ACTUAL

4.  Determine phenotypic ratio and possible genotypes in the following format:

a)  Fraction or percentage of phenotype & genotype b)  ¾ could be Purple (PP, Pp); ¼ could be White (pp)

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Example: Monohybrid Cross

TT x tt T = tall t = short


Yý x Yý Y = yellow ý = green

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Sś x śś S = smooth ś = rough

Example Monohybrid Cross

Purple flowers are dominant over white flowers in pea plants. Cross a Heterozygous purple plant with a white plant.

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Predictions for Two Traits

  Like tossing two coins at once   Outcome of one doesn’t affect outcome of the other

  Involves two separate traits   How many letters will we use this time?

 2 different ones!

  This is called a dihybrid cross.

Rules for Dihybrid Crosses

1.  Figure out all possible allele combinations (next slide) 2.  Place combinations for both male and female on top and

side of 16 square Punnett 3.  Combine parent alleles inside boxes 4.  Determine phenotypic ratios (no need to go through and

do genotypes)

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Practicing Allele Combinations

  AaBb   AB Ab aB ab

  AABB  AB AB AB AB (or just “AB”)

  aabb  ab ab ab ab (or just “ab”)

  AAbb  Ab Ab Ab Ab (or just “Ab”)

Example: Dihybrid Cross

  RrYy x RrYy  R = round; r = wrinkled  Y = yellow; y = green

1.  Figure out allele combinations: a)  1st Plant:

  RY, Ry, rY, ry b)  2nd Plant:

  RY, Ry, rY, ry

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Dihybrid Crosses

2. Place allele combinations on top and side of Punnett Square

3. Fill in boxes

4. Determine Phenotypic & Genotypic ratios

I T ’ S N O T A L L A S S I M P L E A S I H A V E M A D E I T O U T T O B E .

Other Patterns of Inheritance

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Other Patterns of Inheritance

  Not all traits are completely dominant   There are several other patterns of inheritance:

  Incomplete dominance   Codominance   Sex-linked   Polygenic traits   Multiple alleles   Pleiotropy

Incomplete Dominance

  Pattern of inheritance in which heterozygous offspring show a phenotype between the phenotypes of the parents (in the middle)

  NEITHER allele is expressed fully

  Examples:   Snapdragon flowers:

 Red flower + white flower = PINK flower

  Cow color:  Red (brown) bull + white

cow = roan (pink) cow

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  Knowing that a certain flower shows a pattern of incomplete dominance, create a Punnett Square showing a cross of TWO PINK flowers.   r = red   w = white

Codominance

  Pattern of inheritance where both alleles in the heterozygous offspring are FULLY expressed

  Example: Human Blood Type   Genotype = Letters; Phenotype = Blood Type   Type A: AA, AO (homozygous & heterozygous)   Type B: BB, BO (homozygous & heterozygous)   Type AB: AB (ONLY heterozygous)   Type O: OO (ONLY homozygous)

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Codominance

  Knowing that blood type shows a pattern of codominance, cross a person with TYPE O blood and one with TYPE AB blood.

Sex-Linked Inheritance Pattern

  Phenotypic expression of an allele that is dependent on the gender of the individual

  Carried on either sex chromosome (X or Y)   Remember: Female = XX; Male = XY   Many more genes carried on the X chromosome, so many

more X-linked traits than Y-linked traits  Examples: Hemophilia, Color-blindness  FEMALES: If have only healthy X, it dominates over

the infected X.  MALES: If have only one infected X, Y can’t dominate

over it.

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  Knowing that COLOR BLINDNESS is a sex-linked trait, cross a CARRIER FEMALE with a NON-INFECTED MALE.

  Determine the probability of this couple having a color-blind child.

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Pleiotropy

  Single gene affects MORE than one trait   Examples:

  Sickle cell disease   Marfan’s syndrome

Multiple Alleles

  More than two alleles for the same gene   Example:

 Human blood type (phenotypes produced by 3 different alleles)

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Polygenic Traits

  One trait is controlled by TWO or MORE genes   Example:

 Human skin color

Chart that shows how a trait and the genes that control it are passed through a family

Pedigrees

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Pedigree

  Most knowledge of human genetics comes from studying patterns of heredity in populations and families

  Best way to trace these patterns is by creating a pedigree

Pedigree Symbols

  Square = MALE   Circle = FEMALE   Empty shape = does NOT have trait   Half shaded = CARRIER of trait

  Heterozygous individual, carries recessive trait, but doesn’t show it

  Completely shaded = SHOWS trait

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Pedigree: British Royal Family

history of genetics mendelian genetics punnett …

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